I don't know about Acura bose, but on many of the newer GM's, there is not a variable line output from the headunit. It uses a fixed signal, and a data wire to control the volume at the amp itself.

A good quality LOC (like Peripheral SVEN) at the rear speakers is often the easiest and sometimes the only way to go. I don't know anything about the walmart piece, but try to find an adjustable LOC.

Toby